The one-pressing or dual-pressing drain valve is mounted in the bathroom water tank in the domestic or oversea markets, the draining function of the one-pressing drain valve is single, and the dual-pressing is drain valve can achieve two different drainage in one water tank. It wastes a lot of water during flushing process for the water tank with one-pressing drain cannot achieve the control function of switching partial flush and full flush. Two different working water levels of the drain valve decides the control of the drainage of the water tank, such as, for the water tank drainage control of the flapper-type drain valve, the draining water level of the water tank is controlled through opening one of the two seal flappers or opening both seal flappers simultaneously which are arranged with different height in the drain valve, so that the drainages of two water levels of the water tank are achieved, or the counter weight of the flapper is controlled to get different time of that the flappers seals the drain valve outlet base to solve the control of the partial flush and full flush of the water tank; but the ways mentioned above are still the control of the water level in the water tank with complicated structure and low accuracy of switching control of water volume, and it still cannot optimized the utilization of the water resource in the water tank, and still waste water resource.